Click on the gear icon in the Notifications panel and select "Quit Dropbox"
If the issue persists when Dropbox is completely closed, this suggests that the cause is related to something outside of the Dropbox application. You may want to check other applications or components of your Operating System that could be causing this.
If the issue is resolved when Dropbox is completely closed, please let me know.
Apologies for the lengthy response. I hope it helps pinpoint the root cause of the error you're getting.